This is to make explicit the conditional select of libunwind is only
needed for libopenvswitch.  If we spill it over other packages, the
generated kconfig will have recursive dependency issue.  2 new patches
were made for this goal

The other thing is that "+libunwind" will cause it to be built if any of the
packages defined in this Makefile is enabled (y or m).  This is at the moment
by-design of the build system.

Libunwind does not support architectures like arc.  Use conditional select To
avoid (libunwind) build failures like the following,

  checking for ELF helper width... configure: error: Unknown ELF target: arc
  make[3]: *** [Makefile:65: /data/openwrt/build_dir/target-arc_arc700_uClibc/
  libunwind-1.3.1/.configured_68b329da9893e34099c7d8ad5cb9c940] Error 1

Things like "+PACKAGE_openvswitch-libopenvswitch:libunwind" will also result in
recursive deps error for chains of 3 nodes.  Kconfig construct like the
following will be made

	config A
		tristate
		select B
		depends on !(C) || (x)

	config B
		tristate
		select C

	config C
		tristate

	config x
		bool

Other changes include

 - Shared use of variable ovs__common_depends was removed
 - Ovn doc build was patched out

Link: https://github.com/openwrt/packages/pull/12959#issuecomment-665021413
Reported-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

Compared to original packaging scheme within openvswitch, we removed
ovn-trace, ovn-detrace from openvswitch-ovn-common package.  ovn-detrace
requires python libs to run, so it does not actually work out of box in
previous builds anyway.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>